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Fill that hole
Here's how to repair a hole in your boat's hull
by Barry Hammerberg

Sponsored by Good Old Boat Magazine

Bob McPherson gauges the thickness of the laminate, at left, and marks the hole, center. The inside is ground to prepare for the fiberglass patch, right.

A few years ago, a fellow boater abandoned his boat on Lake Michigan as it was sinking in 450 feet of water. Several days later, it washed ashore; the nearly empty fuel and water tanks had prevented it from sinking completely. Examination of the wreck established that an abandoned transducer had dislodged, allowing water to flood the hull at a rate faster than the bilge pump could handle. An extra hole had cost a friend his boat.

The loss was unnecessary. When the transducer was abandoned, the fitting should have been removed and the hull integrity restored. A simple repair by the owner or the yard would have eliminated the risk and protected the hull from a potential source of water damage to the laminate.

Ideally, we'd have no holes below the waterline. Realistically, however, we'd have to give up too much: engines, generators, heads, instruments, air conditioning. So our hulls resemble Swiss cheese.

This doesn't mean we're destined to sink. Proper through-hulls, regularly inspected, are the first line of defense. Seacocks on all hose connections protect against hose failure. Properly sized wooden plugs tied to the through-hulls are the backup in the event of seacock failure. Bilge pumps buy time to react (though few recreational craft have a pumping capacity that will keep them afloat for an extended period). Nonetheless, it's prudent to fill unused holes. It's the most simple and reliable remedy. Yet as a marine surveyor, I've seen everything from disconnected transducers to iron pipe caps on unused through-hulls.

New depth sounder

How simple a remedy? I photographed the process on the hull of a 30-foot sailboat. A new depth sounder was installed last summer. It was smaller than the old one, so the owner elected to drill a new hole and leave the old hole with the abandoned transducer in place. Our fellow boater's story convinced him to complete the job by filling the unneeded hole.
Filling the hole takes the proper materials, tools, and a little time. Robert McPherson, a fiberglass repair guru working in northeastern Wisconsin during the summers and on the East Coast during the winters, enthusiastically agreed to help.

We met at the marina at 8:30 a.m. to start the job. While laying out the tools for the job, Bob said he'd gotten interested in boats and repairs as a young man when Ted Wells, International Snipe Champion in the 1950s, wrote an article on how to bond the seams on a Snipe so it could be dry sailed. Bob covered his wooden Snipe with a canvas soaked with resin.

The tools and materials we'd need that day included acetone, fiberglass cloth and resin, an orbital disk sander, a shop vacuum, and safety equipment such as a dust mask, safety glasses, gloves, and coveralls. You do not want to be sloppy with polymers. Your skin is porous and will absorb these chemicals. Depending on the material you choose to use, you'll risk exposure to epoxy (amino), styrene, benzene, MEKP (methyl ethyl ketone peroxide), and thinners (acetone, MEK [methyl ethyl ketone], and xylene).

Bob elected to use vinylester for the repair, explaining that you could successfully use polyester, vinylester, or epoxy for a small hole like this one. However, vinylester provided a stronger bond than polyester at about half the cost of epoxy. It was also easier to use than epoxy, which he normally reserved for larger structural damage or wood restoration.

Bob makes the pattern for the patch, top. He cuts the fabric and lays out the patch material in readiness for wetting, middle photos. The laminate is wet out, bottom.

The repair's strength comes from the glass fibers. Bob elected to use a product called "tabbing," a knit composite of fiberglass cloth and chopped strand mat. He prefers this for repairs due to its high glass content and stability when wetted with resin. He's found he can wet the material on a piece of cardboard, roll it up, and transfer it to the repair without losing the size and shape of the patch. My experience differs; I try to determine the composition of the original laminate and duplicate it in the patch.

We talked about other reinforcement choices:

•Woven roving -- good strength in two directions but more apt to have voids through the layer.

•Cloth -- finer weave than roving, two-directional, and great tensile strength, but poor layer-to-layer tensile strength due to lack of interlaced fibers.

•Chopped strand mat -- good strength in all directions, but the weakest of the laminates due to the low glass-to-resin ratios attainable; the glass provides the strength, the resin is a binder.

Bob cautioned that the glass should be marine quality. Typically, this means that it has a chrome or silane sizing for better bonding with the polymer. If you choose epoxy for the repair, use stapled, rather than bonded, mat for easier wetting out, since the binder used in mat and some combination fabrics doesn't dissolve readily in epoxy.
For cleaning up, you'll need lacquer thinner or acetone. I normally use acetone as it can also be used to thin gelcoats for spraying. A couple of words about acetone: it has a flash point of 0? F (it can create combustible fumes at temperatures above that), and it evaporates so fast it can cause moisture to condense on a wiped surface. So don't use it to clean a surface just before applying resin. That low flash point also means no smoking.
Meanwhile, back in the boatyard, it was time to start the job. The procedure was straightforward:

Clean the area -- Remove any old fittings along with the associated bedding compound.

Grind it out -- Grind a taper back from the existing holes about 12 laminate thicknesses. Bob uses a small disk sander with a 36-grit open-face disk. The small size of the grinder makes it easier to control, and you can see what you are doing. I noted that Bob also made a lap joint on the inside of the hull. For this, the inside grind is not tapered. Though most of the articles you'll read focus on one side, Bob and I agreed that we slept better when we ground and patched from both sides. Essentially, we created a glass-reinforced polymer rivet in the hole.

Remove moisture -- Check the laminate for moisture. If you have access to a moisture meter, use it. If not, wipe the area with your finger and look for telltale streaking. Dry the area before you start applying resin.

Rebuild the laminate -- You're ready to rebuild the laminate. Bob related some tricks to make the job simpler.

•First, make a pattern, marking it with an arrow for the up orientation. Cut reinforcements of varying diameters starting from the hole and ending up the size of the ground-out area.

•Mix your resin with the appropriate hardener. Take care to follow the mixing directions. Epoxies have very finite mixing ratios; if possible, use metering pumps to get the ratio right. Polyester and vinylester are a little more forgiving; 2- to 8-percent MEKP will do the trick. Too little, and the patch may never harden; too much, and you'll get the same result. Bob uses 12 to 15 cc of MEKP to a quart of polyester or vinylester.

•Wet the laminates on a piece of cardboard, taking care to remove all air, working it to an edge or through the surface of the reinforcements.

Bob McPherson gauges the thickness of the laminate, at left, and marks the hole, below. The inside is ground to prepare for the fiberglass patch, bottom.

•Roll up the patch. This only works if you are using tabbing or a cloth/mat combination. Try to roll up a chopped-strand-mat patch, and you'll have a gumball. Gloves will protect you from the mess and contact with the polymer.

•Lay your internal patch first. If the hole is small, like our through-hull, you can immediately lay the outer patch in place.

•Use masking tape to keep the outer patch from sagging. A plastic spatula works great for shaping the patch through the tape. Mylar film serves the same purpose as the tape but is harder to find. Bob calls masking tape the poor man's vacuum bag.

Use heat if needed -- A heat source may be required if the temperature is below about 50° F. Heat lamps work, but you need to monitor them to prevent overheating and fire. Tenting and heating with a portable heater also solves the problem. I have epoxy-coated keels in 30-degree weather using skirting to create an enclosure for heating.

Cure and shape -- Vinylester laminate will harden in 20 to 40 minutes. Remove the tape and grind to the finished shape. Bob uses an orbital disk with 40-grit closed-face paper.

Fill and fair -- After the shape is proper, fill any voids with compatible marine putty. Do not use automotive body filler; it isn't designed for total immersion. A thin wood spline works great for maintaining the curve of the hull.

Replace the gelcoat -- You're just about done. It's time to replace the gelcoat exterior, your initial water barrier. Mix the gelcoat with a hardener. A ratio of 4 percent works well. I usually add a liquid paraffin wax, as gelcoat is air-inhibited, meaning it stays tacky so a laminate can bond to it. For repair purposes, you want to block the air so the gelcoat completely cures. Paraffin wax floats to the surface and creates a seal and a tack-free surface. Sprayed PVA (polyvinyl alcohol) mold release wax does the same thing, as does a layer of Mylar film. For more on this, see the sidebar on the previous page.

Finish the job -- It's time to finish-sand the patch. In our case, we would be painting the hull, so we sanded the patch with a 150-grit paper to provide a little tooth for the paint and to ensure that all wax was removed. If the boat has a barrier coating on it, it should be replaced before painting.

That's the whole process. By noon the hole was ground, repaired, gelcoated, and ready to paint. This boat no longer had an extra hole.

While the polymer and gelcoat were curing, Bob was off completing other jobs around the yard. I tagged along and picked his brain. I wanted to know how he dealt with larger holes. Sometimes holes aren't left over from some prior use, such as an abandoned through-hull. Sometimes they are unintentionally added. Rocks, docks, debris . . . things that go bump in the night. I asked Bob when you should call in the experts.

Though he makes his living repairing fiberglass, Bob was pragmatic. He allowed that there really wasn't a finite point. It was whenever the job was more than you were comfortable handling. The repair technique isn't all that different than that we'd just completed. The steps generally are:

•Examine and identify extent of damage.

•Determine your access to one or both faces of the laminate.

•Grind away the damaged laminate and taper back about 12 laminate thicknesses.

•Determine laminate construction (a small sample can be burned, if necessary, to remove the resin from the glass layers).

•Determine how you'll support the laminate while it cures. For large holes Bob offered some tricks, like taking a mold off a sister vessel or using Mylar sheets . . . anything that can create a mold shaped like the missing area. The mold is screwed to the hull and the repair is initiated from the inside. After the laminate cures, the screw holes are filled. (For more about a large hole repaired in a shipwrecked Hinckley, see the July 2002 issue of Good Old Boat.)

•Rebuild the laminate using the original construction as your guide.

•Fill and sand any imperfections.

•Gelcoat the exterior/interior; protect both sides from moisture intrusion. Color matching can be tricky and may involve spraying a larger area to get to breaks in the plane of the surface, which tends to hide shade variances. Alternatives include creative graphics or refinishing the hull (or deck) with a marine finish designed for fiberglass applications.

Unlike Bob, I think there are times to call in a professional:
•When the damage is to a structural member, like a stringer or cross frame, and the structural integrity of the original design needs to be restored;

•When the damage is extensive and there is a chance of deforming the fiberglass piece if it isn't properly supported; and

•When the damage is in an area where restoring the finish to match surrounding highly polished or anti-skid surfaces is critical.

Those situations aside, there is no reason why you can't do a professional job of repairing unwanted holes in your fiberglass boat before Mother Nature fills them with water.


The final touch

Making the repair invisible

by Barry Hammerberg

You've finished the first part of your fiberglass repair job; the laminate is strong and fair. Now it's time to hide it. Seems a shame, but if you do a repair job properly, no one should know you did it.

If you're working below the waterline, it is easier to finish the patch. Barrier coats and bottom paints will cover the surface -- you need only to create a smooth seal. If you are working anywhere else, you are faced with duplicating the gelcoat finish and tint.

Either way, the first step is making sure the repaired surface is fair and free of pinholes. I usually use a mixture of silicate filler and gelcoat troweled into the patch with a plastic blade to insure a base for the gelcoat. Sand the area of the patch and the area around it for 1 to 2 inches with paper grit close to 360 to prepare for gelcoat application. Use a block behind the paper to avoid introducing an uneven surface. The final finish will be determined by your base.

Gelcoats are available in a number of formulations based on the resin base: isothalic, orthothalic, neopentyl glycol. Most are air-inhibited, meaning they remain tacky to permit them to bond with additional layers of polymer. Finishing gelcoats that dry tack-free are available. Typically they contain a wax that floats to the surface, sealing it so the surface cures. Cobalt-free gelcoats are also available. They take longer to cure but don't change tint as much when they polymerize. Unless you have an excellent local source of materials, you'll probably buy a "matching" gelcoat from your boat manufacturer, from a boat repair company, or possibly from a chandlery. It likely won't match perfectly, but you may find it acceptable.

Tint with pigments

If you want a better match, you can buy pigments for tinting the gelcoat. Experiment with small samples until you get a satisfactory match. This is where it pays to have an artistic friend used to working with pigments. Check the match by spreading a thin layer of uncatalyzed gelcoat on the surface you are matching. You can remove your uncatalyzed samples with acetone or lacquer thinner. When you are satisfied with the color, tint a larger batch and recheck the match.

I mask off the gelcoat around the patch to prevent overspray, applying the tape at the very edge of the area I sanded. You can brush-apply the gelcoat and wet-sand it to a satisfactory finish. I prefer to spray it, as it takes less sanding, and the finish tends to be less porous. I use the small disposable pressurized sprayers available in most marine supply stores.

For spraying, I catalyze the gelcoat with about 2 percent MEKP by weight, then add a 50:50 blend of acetone and xylene or MEK until the mix sprays smoothly. I use acetone because most of it flashes off in the air after helping to atomize the mixture. The xylene or MEK helps the gelcoat flow out evenly once it's on the surface. I usually don't apply gelcoat if the surface temperature is below 50 degrees, as it takes much longer to set and is likely to sag -- though I've seen professionals apply material when it was in the 40s.

I slowly build up an opaque coating through several applications, often a couple of passes per application, allowing a few seconds for the excess solvent to evaporate. I use air-inhibited gelcoats to facilitate bonding between coats. If you have a finishing gelcoat, you will have to sand between coats. When I have about 10 to 12 mils of material in place, I spray a coating of PVA (polyvinyl alcohol, or mold parting film) over the patch to block air contact and permit a full cure.

When the patch is fully cured (surface doesn't indent with fingernail pressure), I blend and smooth the surface by successively wet-sanding with 400, then 600, paper, taking care to sand the patch back until the gloss line around the edges of the newly applied gelcoat disappears. Again, use a small block to prevent waviness.

Buffing is the final touch. I try to use a buffing compound that complements the color of the patch. If the surface is white, I use a white compound; if the surface is dark, I use a dark compound. I buff carefully to avoid burning the surface. I often mixed my own buffing compounds, adding pigment and paraffin oil to create an approximate match to the gelcoat when I did this in a factory setting. The compound filled any porosity in the factory-applied gelcoat around the patch.

Your patch should be physically invisible, though you may have some tint variation. This can be minimized if you are willing to sand and spray larger areas, extending the refinished surface to features that will hide the tint variation -- stripes, curves, or sharp breaks. When you see a professional refinish a transom, it's frequently because he's elected to use the sharp edge and differing planes to mask color variation. If the areas of repair are extensive, refinishing the entire surface (hull, deck) may be your best route.

A final note: be patient; don't rush the color-matching and curing.

Robert McPherson, Marine repairer
920-991-9374; fax 920-991-1098

For further reading . . .
Don Casey's Sailboat Hull & Deck Repair is available at <> or by calling 763-420-8923.

Author's bio

Don Launer

Barry Hammerberg has boated most of his 62 years. While in high school he rebuilt a Snipe and learned to sail. He was soon building fiberglass canoes and kayaks. Midwesterners, he and his wife, Ruth, owned a charter boat in the Florida Keys and have sailed the BVI and Leeward Islands. They just returned to Wisconsin from a 16-month journey along the Great Loop Route, down the inland rivers, around Key West, up the eastern seaboard, and through the Erie Canal to the Great Lakes.

Sponsored by Good Old Boat Magazine
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